1. Field of the Invention
The present invention relates generally to wireless telecommunication technologies and, more particularly, to digital cordless communication systems including a plurality of stationary base stations for radio communication with multiple mobile communication units and main equipment for interconnection of such base stations to public telephone networks. The invention also relates to digital wireless communication systems with increased frequency stability of radio signals at plural base stations.
2. Description of the Related Art
One typical prior known digital cordless communication system of the type stated above has been disclosed, for example, in Published Unexamined Japanese Patent Application No. 6-318963 ("JP-A-6-318963"), wherein the system is designed to operatively associate with portable mobile communication units (also referred to as "handheld radiotelephone transceivers" or "personal stations" among those skilled in the art) having transmit-and-receive frequency stabilization functions. FIG. 1 depicts a typical electronics configuration of one such personal station.
In FIG. 1 the mobile personal station (PS) is generally designated by reference numeral 60, which is constituted from a receiver circuit 602 which receives a digital radio frequency (RF) signal transmitted over-the-air from one base station--called the "cell station" in some cases--to an antenna 601 and then outputs an intermediate frequency signal, a frequency synthesizer 608 for supplying a local oscillation frequency signal to the receiver circuit 602, a reference oscillator 607 for generation of a signal for use as a reference of frequency at the frequency synthesizer 608, a demodulator 603 that is operable to reproduce or "play back" a carrier wave from the intermediate frequency signal as output from receiver 602 and also demodulate a received signal using as its phase reference signal a certain signal synchronized in phase with the carrier wave, a frequency counter 604 for counting up the frequency of such carrier wave reproduced by demodulator 603, a frequency error detector section 605 that uses the count value of this frequency counter 604 to detect any possible frequency difference or "error" of the reference oscillator 607, and a reference oscillator control circuit 606 responsive to receipt of an error detection signal from the frequency error detector 605 for controlling the oscillation frequency of reference oscillator 607. It is thus possible, based on the frequency of a signal received from the cell station (CS), to achieve frequency stability of those signals as transmitted thereto from one or more mobile PSs.
Additionally, numeral 609 designates a modulator circuit for modulating a transmitted signal by the local oscillation signal fed from the frequency synthesizer 608; 610 denotes a power amplifier for power amplification of the resultant transmit signal as modulated by the modulator 609; 611, a demodulation data monitor circuit of the demodulator 603; and 612, a reference signal control circuit of demodulator 603.
Unfortunately, the prior art frequency stabilization scheme suffers from a problem in that upon failure of accurate synchronization of a reference clock signal in the frequency stabilization control loop employed in wireless CSs, any signal received from a CS is no longer used directly for frequency stabilization of mobile PSs because of the fact that the conventional system is inherently designed to effectuate intended PS frequency stabilization based on such received signal only.